Field of the Invention
The present invention relates to an image forming apparatus.
Discussion of the Background
An image forming apparatus operates to cause a paper feed unit to deliver stored recording papers one by one to a conveyance path by paper feed rollers, and then cause conveyance rollers to conveyance the recording paper to an image forming unit so as to form an image on the recording paper. The image forming apparatus has timing rollers disposed at a position in front of the imaging forming unit on the conveyance path, and the timing rollers are to be driven to ensure that the recording paper is conveyed to the image forming unit in synchronization with image formation timing in the image forming unit. Conventionally, the image forming apparatus includes several sensors to detect the state of the recording paper on the conveyance path in order to ensure stable execution of the conveyance of the recording papers on the conveyance path (see Japanese Unexamined Patent Application Publication No. 07-319241).
Japanese Unexamined Patent Application Publication No. 07-319241 has proposed an image forming apparatus including a recording paper presence/absence sensor to detect the presence or absence of the recording paper in the paper feed unit, and a recording paper length detection sensor to detect the length of the recording paper which is disposed on the downstream side of the recording paper presence/absence sensor. The image forming apparatus according to Japanese Unexamined Patent Application Publication No. 07-319241 allows the paper feed rollers to keep rotating based on recording paper length information during the time that the recording paper presence/absence sensor detects the presence of the recording paper. On that occasion, the paper feed roller on the upstream side is allowed to keep rotating so as to assist the conveyance of the recording paper even in the state in which the fed recording paper reaches the conveyance rollers located on the downstream of the paper feed rollers.
In the conventional image forming apparatus, the rotational velocity of the paper feed rollers is set higher than the conveyance rollers so as to feed the recording papers of different types and thicknesses, thereby configuring so that the fed recording paper surely reaches a conveyance sensor close to the conveyance rollers within a predetermined period of time. With the image forming apparatus, when the conveyance sensor detects the front end of the fed recording paper, the rotational operation of the paper feed rollers on the downstream side is suspended to inhibit excessive conveyance of the recording paper by the continuous rotation of the paper feed rollers, thereby preventing the occurrence of damage, such as paper folding.
When the rotation of the paper feed rollers is suspended at the timing at which the conveyance sensor detects the front end of the recording paper in the conventional image forming apparatus, the paper feed rollers are to be suspended in the state in which the recording paper being conveyed by the conveyance rollers is caught in the paper feed rollers. Therefore, the load exerted on the conveyance rollers is increased by the amount of frictional resistance of the recording paper caught in the paper feed rollers, and a slip may occur between the conveyance rollers and the recording paper. The occurrence of the slip may cause a delay in the conveyance velocity of the recording paper, thus leading to the occurrence of a timer jam (virtual paper jam due to the passage of time) or deterioration of productivity in the recording paper conveyance. With this image forming apparatus, a recording paper detection sensor disposed on the conveyance path is to measure passage time of the recording paper. When the passage time is longer than a predetermined period of time, a determination is made that the timer jam has occurred.
With the configuration of Japanese Unexamined Patent Application Publication No. 07-319241, the amount of feed of the recording paper by the conveyance rollers on the downstream side is decreased when the slip of the recording paper occurs on the conveyance rollers on the downstream side. On that occasion, the paper feed rollers on the upstream side perform the rotational operation only a predetermined period of time determined with the recording paper length information, so that the rotational operation of the paper feed rollers may be suspended during the conveyance of the recording paper by the paper feed rollers. Consequently, the amount of slip on the conveyance rollers is increased, and the amount of movement (velocity) of the recording paper is further reduced (decelerated). Hence, even with the configuration of Japanese Unexamined Patent Application Publication No. 07-319241, the occurrence of the slip contributes to the occurrence of the timer jam or the deterioration of the productivity in the recording paper conveyance.
In view of the foregoing problems, the present invention has an object to provide an image forming apparatus capable of stably conveying the recording papers regardless of the presence or absence of the occurrence of a slip during the recording paper conveyance.